1. Field of the Invention
The present invention relates to a method of forming the device isolation structure of an integrated circuit (IC). More particularly, the present invention relates to a method of forming a shallow trench isolation (STI) structure.
2. Description of the Related Art
A complete integrated circuit usually consists of millions of metal-oxide-semiconductor (MOS) transistors. To prevent any two neighboring transistors from short-circuiting, some isolating structures need to be formed between transistors. Conventionally, a field oxide (FOX) layer is formed around transistors by local oxidation of silicon (LOCOS). However, due to many side effects related to the use of the field oxide layer such as internal stress and bird's beak encroachment problem, field oxide isolation has been gradually replaced by shallow trench isolation (STI) structures. In fact, as the critical dimensions (CDs) of devices continue to decrease, STI structures have become widely used in deep sub-micron fabrication process.
FIGS. 1A through 1C are schematic, cross-sectional views showing the steps for producing a conventional STI structure in a substrate.
As shown in FIG. 1A, a pad oxide layer 102 and a mask layer 104 are formed over a substrate 100 in sequence. The pad oxide layer 102 is formed by thermal oxidation. A shallow trench 106 is formed in the substrate 100 by a photolithographic and etching process. A liner oxide layer 108 is formed on the exposed substrate 100 inside the shallow trench 106 by thermal oxidation.
As shown in FIG. 1B, an oxide layer 110 that fills the shallow trench 106 is formed. The oxide layer 110 is formed by depositing oxide material into the shallow trench 106 and over the mask layer 104 to form an oxide layer (not shown in the figure). The oxide layer above the mask layer 104 is then removed by chemical-mechanical polishing. The oxide layer 110 is formed by using ozone (03) and tetra-ethyl-ortho-silicate (TEOS) as reactive gases in a chemical vapor deposition (CVD).
As shown in FIG. 1C, the pad oxide layer 102 and the mask layer 104 are removed to form the STI structure. The pad oxide layer 102 and the mask layer 104 are removed by wet etching.
Because a shallow trench has a definite aspect ratio (AR), in other words, depth of the shallow trench is greater than its width, filling the shallow trench with oxide material become increasingly difficult as the critical dimensions of devices are reduced. This is especially serious when O.sub.3 and TEOS are used as the reactive gas in a chemical vapor deposition to form the oxide layer in the shallow trench. Since the deposition rate of oxide on the bottom and the sides of the shallow trench are almost identical, after filling the shallow trench with oxide material, voids or seams are likely formed in the middle of all shallow trench structures with a definite aspect ratio.